1. Field of the Invention
The present invention relates to a piston apparatus of an automatic transmission.
2. Description of the Related Art
In a vehicle including an engine (internal combustion engine), an automatic transmission that automatically optimally sets the transmission gear ratio between the engine and a drive wheel is used as a transmission that appropriately transmits torque generated by the engine and rotation speed to the drive wheel in accordance with the running condition of the vehicle. One example of such an automatic transmission is a multi-stage automatic transmission that sets the transmission gear ratio (gear ratio) with use of a friction engaging element such as a clutch or brake, and a planetary gear apparatus.
Such an automatic transmission is provided with a hydraulically actuated piston apparatus in order to engage and release the friction engaging element such as a clutch or brake. Conventionally, a so-called push-type piston apparatus has been used as such a piston apparatus, but in recent years, so-called pull-type piston apparatuses have been used in order to reduce the size of the automatic transmission. As disclosed in, for example, JP 2005-320990A, a pull-type piston apparatus includes a cylindrical piston member (actuating member) that can move in the axial direction of the rotation shaft of an automatic transmission, and a hydraulic chamber. The pull-type piston apparatus is configured so that the piston member is moved in the axial direction by controlling hydraulic fluid (ATF: Automatic Transmission Fluid) supplied to the hydraulic chamber. With such a pull-type piston apparatus, when engaging the friction engaging element, pressure in the hydraulic chamber is transmitted to the piston member, and thus the piston member moves as if it were being pulled to one side in the axial direction. Then, the friction engaging element is pressed by a pressing member that is provided integrally to the piston member. In this case, the hydraulic chamber and the pressing member are disposed on opposite sides of the friction engaging element in the axial direction, thus sandwiching the friction engaging element.
However, with the piston apparatus disclosed in JP 2005-320990A, the pressing member that presses the friction engaging element is a separate member from the piston member. For this reason, there has been the problem of a large number of parts. Also, there has been the problem that it has been necessary to fix the pressing member to the piston member with use of a snap ring or the like.
For this reason, for example, a piston apparatus in which the pressing part that presses the friction engaging element is formed integrally with the piston member, as shown in FIG. 5, has also been used. The following is a specific description of the piston apparatus shown in FIG. 5. FIG. 5 shows part of a piston apparatus for a clutch of a conventional automatic transmission. This piston apparatus includes a piston member (actuating member) 141 that can move in the axial direction due to pressure in a hydraulic chamber not shown in FIG. 5.
Firstly, as shown in FIG. 5, a clutch C used as the friction engaging element is constituted by multiple outer clutch plates P1 and multiple inner clutch plates P2 disposed between the outer clutch plates P1. The outer clutch plates P1 are spline-fitted into the inner circumferential face of a cylinder part D1 of a clutch drum D. The inner clutch plates P2 are spline-fitted into the outer circumferential face of a clutch hub H.
The piston member 141 includes a cylinder part 141a that extends in the axial direction and to which pressure in the above-described hydraulic chamber is transmitted, a pressing part 141b that extends in the radial direction and presses the clutch C, and a connecting part 141c that connects the cylinder part 141a and the pressing part 141b. A face 141e of the pressing part 141b on the clutch C side is a pressing face that presses against an outer clutch plate P1 of the clutch C. The connecting part 141c is constituted by a first portion 141f that extends inward in the radial direction (toward the center of rotation) from an end of the cylinder part 141a, and a second portion 141g that extends so as to join the inner circumferential edge of the first portion 141f and the outer circumferential edge of the pressing part 141b. Also, in order to prevent the connecting part 141c of the piston member 141 from interfering with the clutch drum D when engaging the clutch C, the connecting part 141c of the piston member 141 is configured so as to be, with respect to the axial direction, offset from the pressing part 141b in the direction of separation from the clutch C.
With such a piston apparatus in which the pressing part that presses the friction engaging element is formed integrally with the piston member as described above, from the viewpoint of reducing the size of the automatic transmission, the axial direction offset amount (L2 in FIG. 5) of the piston member is set so as to substantially match the piston stroke (ST2 in FIG. 5) of the piston member. In the configuration shown in FIG. 5, the offset amount L2 is an axial direction offset amount of the connecting part 141c with respect to the pressing part 141b of the piston member 141. Specifically, the offset amount L2 is the axial direction distance between the pressing face 141e of the pressing part 141b and a wall face 141h of the first portion 141f of the connecting part 141c that is on the clutch C side. The piston stroke ST2 is the axial direction distance that the piston member 141 can move between the released state and engaged state of the clutch C. Note that in FIG. 5, the position of the piston member 141 when the clutch C is in the released state is indicated by solid lines, and the position of the piston member 141 when the clutch C is in the engaged state is indicated by dashed double-dotted lines.
However, there are the following points of concern with configurations such as the configuration shown in FIG. 5. Specifically, in the automatic transmission, when engaging the clutch C, the piston member 141 strokes to one side in the axial direction (in the Y1 direction in FIG. 5) when pressure in the hydraulic chamber is transmitted to the cylinder part 141a. Accordingly, the pressing part 141b of the piston member 141 presses an outer clutch plate P1 of the clutch C. At this time, a load (bending moment) M2 directed toward the other side in the axial direction (in the Y2 direction in FIG. 5) acts on the pressing part 141b of the piston member 141. In this case, stress becomes focused on a bending portion A2 that connects the cylinder part 141a and connecting part 141c of the piston member 141, thus causing a problem with the durability of the bending portion A2.